Going Beyond 2x4 Walls in a Warm Climate

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This question comes up fairly often in our HVAC design work. Clients want to know not only about how much energy they'll save but also if they'll be able to downsize the HVAC system. The former saves on operating cost, the latter on first cost. Let's take a look.

Total R-value

A wall assembly is a combination of materials that allows heat to flow in both series and parallel. For this article, I'm going to show you the results I got for four different wall assemblies. First, I've set up a spreadsheet that calculates the total R-value of an assembly and then used that tool to calculate the R-values for these four walls. Here's a summary of them (assuming grade 1 installation):

Descriptions of walls modeled for exterior insulation vs. thicker walls

The first one is your standard 2x4 wall with plywood or OSB sheathing. It's also based on wood studs, not steel. The second one is the base wall plus half-inch, R-3 continuous exterior insulation. The third one is the base wall plus two-inch, R-10 continuous exterior insulation. The last one is a 2x6 wall with R-19 cavity insulation and no continuous exterior insulation.

The best R-value is the one with two inches of exterior insulation. The 2x6 wall is roughly equivalent to a 2x4 wall with a half-inch of exterior insulation.

HVAC design loads

To see what effect these different R-values have on the size of the heating and cooling systems in a home, we have to put in actual numbers for a house. I chose one that we worked on recently. It's a typical house: two stories, 2,800 square feet of conditioned floor area, and about 2,400 square feet of wall area. I've got the file set up in RightSuite so I looked at the heating and cooling loads for the different walls shown in the table above.

But I also looked at the results for three different locations in 2x4 world: Decatur, Georgia (in the Atlanta area and where the house really is located), Dallas, Texas (a hotter climate), and Phoenix, Arizona (a climate so hot I don't know why people live there but hey, that's their choice). Here's what I got:

Design loads for 2x4 walls with and without exterior insulation and a 2x6 wall without exterior insulation

The design temperatures for the three locations were:

Decatur: 25° F & 91° F

Dallas:   28° F & 99° F

Phoenix: 42° F & 108° F

The results here show that the most reduction we can get in the design loads is about 4,000 BTU/hr. That's about a third of a ton of cooling capacity. In other words, you're not going to find much reduction in your equipment costs by adding exterior insulation or going from 2x4 to 2x6 walls. Dropping three to four thousand BTU/hr may allow you to move down one size in your equipment. Maybe.

Energy savings

Well, if you're not going to save much on the first cost of your heating and air conditioning equipment, what about the energy you'll save each year by reducing the heat transfer across your walls? It's not hard to calculate how much heat flows across a wall over the course of a year. The formula for annual heating load is:

Heat flow equation for annual load based on heating heating degree days


For the three locations I'm looking at, the number of degree days are:

Decatur: 2,671 HDD & 688 CDD

Dallas:   2,192 HDD & 1,346 CDD

Phoenix: 923 HDD & 2,932 CDD


Annual loads for 2x4 walls with and without exterior insulation and a 2x6 wall without exterior insulation

Note that I added another assembly here. In addition to the four above, I included the 2x4 wall with one inch of continuous exterior insulation with an R-value of 5. I figured someone was going to ask about it in the comments.


But if we look at the percent reduction in heat transfer through the walls, it's significant. The 2x4 wall with R-3 exterior insulation and the 2x6 wall both reduce the heat flow through walls by about a quarter (22% and 25% respectively). Going to the 2x4 wall with R-5 exterior insulation reduces your wall heat transfer by about a third. Going to two inches of exterior insulation on a 2x4 wall cuts your annual heat transfer through the walls in half.

Is it worth it?

The 2x4 wall with two inches of exterior insulation gets you the best results among the five assemblies I included here. You could, of course, add even more exterior insulation and reduce the heat transfer even more. Or you could go to a fatter wall without exterior insulation, such as a Joe Lstiburek's ideal double-stud wall. Keep in mind, though, that each inch of thickness you add to the standard 2x4 wall increases your cost, and each successive inch buys you less and less reduction in your wall heat transfer.


If you're in a warm climate and want to go beyond a standard 2x4 wall, you can do this analysis for your area and with your local energy costs to see if it's cost-effective for you. Of the four improved walls above, the one that's going to be the lowest cost is probably the 2x6 wall. It's also the safest to build if you've got a framing crew that hasn't done exterior insulation before. I don't mean that your house could fall down if you build it with exterior insulation. I'm saying crews that have never done window bucks or attached siding over exterior insulation may have trouble with the details. But they can frame 2x6 walls.

There's a learning curve with exterior insulation. If you have a crew that can deal with 2" continuous exterior insulation, though, you can cut the heat transfer through your walls in half and still end up with walls the same thickness as 2x6 walls.

Other reasons to go beyond the 2x4 wall

For some people, the cost-effectiveness based on current energy costs determines whether an improved wall makes the cut. Or maybe it's just how many dollars it adds to the bottom line. But let me suggest some other reasons to consider an improved wall over a standard 2x4 wall.

First, there's comfort. Whether you go with a 2x6 wall or a 2x4 wall with exterior insulation, that extra insulation means that the inside surfaces of your exterior walls won't be as cold in winter or as warm in summer. Now, you may think that doesn't matter a whole lot as long as your heating system or air conditioner is up to the task, but you'd be wrong.

As it turns out, air temperature is only one factor that affects our comfort. Another really important one is called mean radiant temperature. Anyone who's ever walked around naked in poorly insulated house on a cold day has a visceral understanding of mean radiant temperature. Even if the air temperature is warm, cold walls (or windows) can suck the heat right out of you and leave you chilled. (My friend Robert Bean is the most peristent advocate for comfort I know. If you want to do a deep-dive into this topic, check out the pages on mean radiant temperature on his website, Healthy Heating.)


This article is targeted toward warm climates, so moisture accumulation in cold sheathing isn't that big a deal. The colder your winters are, though, the more important it is to keep your exterior sheathing dry. Exterior insulation over the sheathing is great for that. One of the weaknesses of double-stud walls is that the sheathing stays colder and thus wetter, possibly even wet enough in cold climates to cause damage. (See my article on Building Science Corporation's research on a house with double-stud walls.)

Where you may find more savings

If I were building a house for myself here in the mild climate of Georgia and were choosing from the wall assemblies above, I'd go with the 2x4 wall with 2" of continuous exterior insulation. I like the idea of cutting my wall heat transfer in half. But I'd also make sure I wasn't do stupid things to negate the savings from the walls.


  • Windows - A good window is still a poor wall. A window that meets code here in Georgia has an R-value of about 3. That's a heck of a lot less than the R-23 wall I just said I'd build, so I'd do two things. I'd install windows a good bit better than R-3. And more important, I'd make sure I didn't have silly solar gain problems. Windows can kill you on your cooling loads, so you've got to pay attention to placement, overhangs, and sizes.
  • Ducts - First of all, I'd have no ducts in the attic. Then I'd have no ducts in any other unconditioned spaces. Then I might even have no ducts at all, at least for heating and cooling. And whatever ducts I do have would be designed properly, installed well, and commissioned completely.
  • Foundation - A lot of comfort and moisture problems originate with the foundation, whether it's slab-on-grade, crawl space, or basement. The details matter. Even in a hot climate like central Texas, slab-edge insulation can save you a lot on heating. We've got a client there building a Mediterranean style house with a lot of exposed slab perimeter. Slab-edge insulation would reduce their heating load by 25%. (And believe it or not, their heating load is higher than their cooling load.)

Whether or not you go for an improved wall assembly in a warm climate depends on your particular priorities. It's not the slam dunk it is in a cold climate, but there are benefits.


Related Articles

The Diminishing Returns of Adding More Insulation

The Layers and Pathways of Heat Flow in Buildings

The Fundamentals of Heating and Cooling Degree Days, Part 1




Your Total (whole wall) assemblies don't make sense to me. If the 2x4 total R-value is 11.8, and you add R-3 of continuous insulation, the total R-value should be 11.8 + 3 = 14.8. You show 15.2, a difference of 3.4. Then for the 2" continuous insulation of R-10 you show 22.6. A difference of 10.8.

1. Why is the 1/2" R-value = R-3, or R-6 per inch and the 2" R-value = R-10, or R-5 per inch. Why is different insulation used?


3. If you are adding a "skin effect", why is it not identical in each wall?



Any comment about how termites like to live in slab insulation? I know DOE had awarded a grant on doing some research into termite resistant slab insulation but I confess I lost track of what the results of it. This was one of the first things I learned when I first started working in building science at energy extension - termites didn't eat insulation but sure loved to live in it.


Well, I did mention slab-edge insulation at the end of the article, Arlene, but I didn't want to go too far down that termite hole. Definitely a topic I need to cover in a future article.



I'm not generally a fan of ICF construction, but I'm curious how this system mitigates termite risk. I recall some years ago an ICF builder told me they use chemically treated EPS for basement walls and foundations, stem walls, etc. So I'm also curious about research on how this has worked out.


It is an important rabbit hole. Energy doesn't exist in a vacuum. One always has to be mindful of unintended consequences. Now, not every warm climate is a bastion for termites, but there is a large area where you can't get away from them.

So maybe not a rabbit hole, but a termite tunnel?

There's a ICF manufacturer which contains termite treated foam. Allegedly it's the only "code approved" ICF product for use in termite zones. *shrug*

I am building in Blairsville GA at 2300 ft on mountain ridge facing ENE over Lake Notterly. Would love your advice. Using 2X6 framing with 2" closed cell, 1" exterior Zip-R and Rockwoll. Builder says way overbuilt. Also, I insulated slab which he never does!



And yeah, that's why I called it a termite hole.

@Richard: I concur with the builder. The Zip-R minimizes thermal bridging, which is a big deal. But the main benefit of cavity foam is air sealing (plus a small bump in R-value in the case of ccSPF). You can seal the cavities MUCH cheaper (and equally effective) by 'picture framing' all the sheathing-stud intersections with caulk or closed cell foam (via Hilti type applicator). You'd get a lot more benefit by upgrading your sheathing to 1.5" Zip (R-6.6).

As for cavity insulation... unless you have another reason to use Rockwool, I think you'd get more bang for the buck with blown-in insulation (Spider, BIBS or cellulose).




Good analysis, Thanks... How about a 2x6+3 and a 2x6+5 walls? These are more typical upgraded walls.
1. Per ACCA MJ8, aren't Dallas design conditions 24/98? Why did you use 28/99?


Armando, yeah, it's easy to do the analysis for 2x6 +3 and 2x6 +5, but in warm climates, that takes you well into the diminishing returns regime.


2. Yes, termites are definitely an issue with slab insulation in warm climates. That doesn't mean it's impossible to do.

1. Can you please expand on diminishing returns? Thermal bridging doesn't save that much energy in hot summers? Another benefit for using outsulation is that it makes an additional moisture barrier when taped with an approved tape.


Cause North Dakota is a well known location in a warm climate...

How would this apply to a northern location?


Matthew, see my comment to Gary Reecher just above yours.



Yes indeed! There's SIPs, ICFs, double-stud walls, and more. I'll get to some of those in the future.

Allison, I was surprised to see you using only cavity R values in your charts - this can be quite misleading, as you know DOE studies have shown framing factors of 2x4 stud walls are 25-27% - and this is BEFORE building codes strengthened the shear wall standards in 2012. So your numbers are off by quite a bit. I would recommend a 4.5" thick wall with R26 continuous (except at framed openings and top & bottom plates) closed cell foam. Joe Lstiburk refers to this as the "Clever Wall" - it involves MUCH less labor & materials - and opportunities for error (OFEs - remember these are manufacturing sites prone to error!) - than the double stud wall (or single stud wall for that matter). I know of homes in South Texas - and Columbia SC - built this way on a protected concrete slab that is being passively cooled by the earth.


Charles, those aren't just the cavity R-values. They're the total R-values that include air films, drywall, sheathing, framing, cavity insulation, and continuous insulation based on a framing factor of 23%. I'm not sure why you read it that way.


I had a similar thought when I was reading it, but came to the conclusion that you're listing nominal cavity R-values for simplicity and allowing your manual J calculator to factor in the entire assembly. Is that correct?


Yes, Bob, that's correct. Sorry for the confusion.

Those heat transfer equations are idealized to assume zero heat transfer via air infiltration/exfiltration? If yes, I'd guess this would significantly reduce those percentages. Are there studies that try to quantify the actual real-world air transfer typical of homes (varying significantly with build style and build date, no doubt)? Leaky windows, doors, drywall, floors, etc.. Opening the door 10 (or maybe it's closer to 100? 1000?!) times a day, seems like a crapshoot to estimate this!
Would be interesting to see a multi-year study of a specific home's actual heating/cooling usage, compared to the theoretical calculations for that specific home, in that specific climate. this might at least allow us to lump together and quantify all the real world Chinks in the proverbial Armor



Is there any reason to adjust for walls being beaten with sun? Seems to me that while the air temperature might be 100deg, the surface temp of a wall hit with sun is much higher. You mention "solar gain problems" with windows, but don't they also apply to walls (albeit less so).




I was lucky enough to move to Richmond VA and we seem to have the best / worst of both worlds. From what I can see, we're almost 50/50 on HDD/CDD and I'm trying to extrapolate your figures for heat, cold and humidity. It would seem that external insulation wrap would still be optimal as it 'works' well for the heat, cold and humid elements. Would you be inclined to agree?
Is thermal mass inside insulation a topic for modern American building methods? I've some sustainable building experience and rammed earth, stone etc inside insulation has some research pointing to benefits.

Whether 2x4 or 2x6, 16"oc vs 24"oc and aligned studs with single plates, all makes a big difference in how much cavity insulation there is and heat transfer through the wood.

2" exterior insulated sheathing presents difficult (and more expensive) installation issues. Fasteners for the siding have to penetrate the studs (how much depends on the type of fasteners and the wind load zone you are in) but at a minimum you are over 3" if you leave a proper drainage plane. There are ways to do it but I think a better alternative to the 2x4 with 2" of foam is a 2x6 with 1" of foam. Though it is a slightly lower performing wall it doesn't present as much of an installation problem.


Have you seen the "production builder friendly" insulated wall? 2x6 top/bottom plates then vertical 2x4s are nailed flush with the interior edge of the 2x6's, then sheathing on the exterior side of the 2x4 and a layer of rigid foam on the exterior side of the sheathing. The idea is that offsetting the 2x4 enables the rigid foam layer to remain co-planer to the slab edge all the way up to the roof.

I'm most likely the central Texas client that you referred to at the end of your article. Termites ARE the reason that I didn't insulate the slab perimeter. They are rampant here, building dirt tunnels 4-5 inches high in the grass. I wish that I could have insulated the slab, but it's just not worth the risk (to me). BTW our electric bill for July was only $140 for our 2800 sqft house. That was the same amount we paid in our 600 sqft rental while we were building. I'm a very satisfied and happy client.


Nice to hear from you, Gary. Although I guess I could have been referring to your home, the one I was talking about in the article was actually a job we're working on now in the Hill Country near New Braunfels. Termites are definitely an issue, but that's not why they didn't do slab-edge insulation. The recommendation came to late for them as they already had their walls up. Glad to hear about your low cooling bills!

I look at this differently... a 2x6 wall 24-oc is not much more expensive than 2x4 16-oc including insulation. When you do optimal-value engineering, it's clear that 2x6 24-oc should be the baseline for all other assemblies.

Then the question becomes how much exterior insulation? In cooling dominated climates, anything beyond 1" is usually not justified (depends on several factors). Moreover, with stucco cladding, 1" EPS is built into the price, and with other cladding types, a 1" layer of EPS or XPS can readily be accommodated.

The challenge is getting builders on board with 24-oc and advanced framing techniques in general. The vast majority of homes built in my area over the past 12 to 15 years have 2x6 frame walls spaced 16-oc. That makes no sense!

I'm with you David. 24" centers and other advanced framing are the way to go. I hope we can get this done on the replacement homes from the fires in northern and southern CA.




If it makes any difference to wall performance... what air infiltration value was used?


Thanks, Brad!

I didn't include the effects of infiltration in these results. That would be a bit beyond the simplistic calculations I did here. That's a job for a more robust hygrothermal modeling tool, like WUFI.

Thanks for the link to the WUFI post. My edu-macation continues... :-)

Regarding framing, the comment states you used a framing factor of 23% in all walls in the comparison. Does that translate into the 2x4 wall @ 16" oc and the 2x6 wall @ 16" oc? Since 2x6 walls are usually framed at 24" oc, what framing factor does that translate into?
Is the framing factor just the percentage of wood in the assembly, or does the placement of the wood affect it? What would the framing factor be on a 2x4 wall @ 12" oc but offset every other stud with 2x6 sill and top plate?

RE: Insulation: Isn't it possible to get more than R-19 in a 2x6 cavity?

@Cal, 23% implies more precision than is due. Real-world framing factors vary several percentage points depending on number of corners, wall and window heights, and framing details (notably advanced framing). IIRC, the framing factor for a 2x6 wall 24-oc ranges from about 10 to 15 percent. A 2x6 wall 16-oc has the same FF as 2x4, although the u-value would obviously be different.

The framing factor for a "2x4 wall @ 12" oc offset every other stud with 2x6 sill and top plate" would be similar to
2x6 24-oc (10% to 15%) since the 2x4 studs are not thermal bridges. The 2x4 studs would increase the assembly u-value. Not sure what purpose they serve...?

High-density batts for 2x6 framing are rated at R-21. I believe 2x6 BIBs and Spider(tm) is rated at R-23. Flash-and-batt or flash-and-blow with a layer of closed cell foam would be a bit higher than that.


Cal, David is correct that the 23% number implies greater precision than really exists. The reason I used that number is that it's the one RESNET uses in the energy modeling for home energy ratings. Here's the part of the table from their standard showing wall framing factors:

RESNET 301 framing factor table

By this table, the answers to your question about walls framed at 24" on center would be 20% for standard framing and 16% for advanced framing. I'm not sure where David's numbers of 10-15% come from so maybe he'll let us know. It doesn't seem likely to get a 10% framing factor with 24" on center.

For example, in a 10' section of wall, 8' high, there would be 6 studs and 3 plates. That would give you a 12% framing even without openings and no corners. To get down to a framing factor of 10%, you'd have to do something like structural insulated panels connected with wood splines 48" on center.

Those are 'default' values based on conservative assumptions within each category. I've seen research that shows ranges found in practice, which is presumably how RESNET arrived at that table. As you would expect, the ranges fall above and below the RESNET defaults.

Following your example... what if walls are 10' with single top plate and only 4 corners? Also, keep in mind that with advanced framing, windows framed without jack studs reduce the overall framing ratio, especially when aligned on a 24 inch module with studs, and headers sandwiched with rigid insulation. I can't find my reference at present. The lowest framing factor for 24 oc may be a bit higher than 10% but you can definitely do better than 15%.


Yes, actual framing factors can be all over the place. In homes designed without any thought given to the enclosure, they can be significantly higher than the defaults in the table above. Some of the designs I've seen have corners all over the place, too many windows, and headers where they aren't needed. Then the framers show up without a detailed framing plan to work from, and the result is a thermal disaster.

Now about this: 


I'd agree with that statement if you put the word "can" right before "reduce." Smaller windows and uninsulated headers will result in higher framing factors. But if someone is doing advanced framing and eliminating jack studs, yes, in general windows should reduce the framing factor.

Great article! I've been really impressed with the quality of articles on your blog. Keep up the great work.

I would love to see the comparison in cost and performance between the 2x4 wall at 16" o.c. 25% framing factor with R13 in the cavity vs. a 2x6 wall at 24" o.c. 15% framing factor with R19-R21 in the cavity. Adding your choice and quantity of exterior foam on top of either would cost the same from there. How about coming up with the cost of that 10' section of wall talked about, one 2x4, one 2x6 OVE? Great discussion, I always learn from these great articles!


Stay tuned, Dave. That's the direction I'm going in this little series.

Good Stuff!

Good stuff

What would be the best for a home in GA? 2x4 or 2x6 walls? Which would be for interior and which for the exterior?

I recommend 2x6 exterior walls in most climates, including yours, but only if spaced at 24 inches on-center. In that case, there's not much, if any, additional cost for 2x6. Interior walls are virtually always 2x4 unless there's a structural reason to go larger. Sometimes walls behind certain plumbing fixtures are 2x6.

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